Electromagnetic Induction practice quiz

Key ideaΦ_B = ∫B · dA = BA cos θ for a uniform field, where θ is measured from the area’s normal.

Common mistakeUsing sin θ by measuring the angle from the surface instead of from its normal.

Key ideaε = −N dΦ_B/dt: an emf appears only when the flux is changing, whether B, the area or the angle changes.

Common mistakeThinking a large but steady flux induces an emf.

Key ideaThe induced current opposes the change in flux: it adds field when the flux is decreasing and opposes the field when the flux is increasing.

Common mistakeMaking the induced field oppose the external field itself instead of its change.

Key ideaA conductor moving through B has ε = BLv (with B, L and v mutually perpendicular). The resulting current feels a force that opposes the motion.

Common mistakeGetting the current direction or the retarding force direction wrong. Use Lenz’s law to check.

Key ideaAn inductor opposes changes in current: ε = −L dI/dt. It stores energy ½LI².

Common mistakeThinking an inductor opposes current itself. It only opposes changes in current.

Key ideaτ = L/R. Right after a switch changes, the current through an inductor keeps its old value; long after, the inductor acts like a wire.

Common mistakeLetting the inductor current jump instantly when the switch is closed or opened.